An apparatus of this type is known from EP 2 103 412 A1 (corresponding to US 2009/0236032 A1). In this case, a channel for the supply of support air opens into the space between the external nozzle and internal nozzle, it being possible to switch the support air on and off. A separate venting channel, which is constantly open to the atmosphere, in turn opens out of the region between the external nozzle and internal nozzle.
An apparatus of a similar type is known from U.S. Pat. No. 5,320,797 B1, in which the venting in each case takes place by opening a solenoid valve arranged in the channel for the supply of support air, the supply of support air being interrupted.
The configuration with a constant connection to the atmosphere has basically proven to be advantageous in the production of large twin wall pipes made of polyolefins, in other words polyethylene and/or polypropylene. Large twin wall pipes are taken to mean here pipes with a nominal width ≧DN 250, the twin wall pipes comprising simple twin wall pipes and twin wall pipes with a double-layered integral socket.
It has been found that this system is still in need of improvement.